The present invention is particularly applicable but not necessarily limited to a process for producing corrosion resistant terne plated copper brazed multi-ply steel tubing of the general type as shown and described in U.S. Pat. Nos. 1,892,607; 2,092,557 and 3,875,027, all of which are assigned to the assignee of the present invention. Multi-ply steel tubing of the foregoing type is in widespread commercial use, particularly for fabrication of automobile hydraulic brake lines and fuel lines. The multi-ply copper brazed steel structure of such tubing provides for excellent mechanical properties and formability, as well as a high resistance to vibration fatique. Further improvements in the resistance to corrosion of multi-ply tubing of the foregoing type has heretofore been provided by applying a copper and/or nickel electroplate to the surface of the welded multi-ply tubing, followed by a coating of a hot terne alloy in accordance with the procedure as more fully described in U.S. Pat. No. 3,875,027, the substance of which is incorporated herein by reference. Processing difficulties have been encountered in producing such corrosion-resistant tubing in accordance with the aforementioned patented process due to the presence of minute particles of carbon embedded in the surface of the copper coating on the brazed tubing, detracting not only from attaining a uniform impervious copper and/or nickel electroplating, but also causing a contamination of the nickel and copper plating solutions, whereby less than optimum results are obtained. The presence of such carbon particles on the tube surfaces and the contamination of the electrolyte has necessitated the use of relatively slow production rates to achieve satisfactory electrodeposited coatings, which has detracted from the economics of the manufacturing process.
The presence of such minute carbon particle contaminants on and embedded in the surface of the outer copper layer is derived from the deposition of carbon or a carbonaceous material on the surface of the tubing prior to the furnace brazing operation, forming a carbon film which retains the molten copper layer in position on the tubing surfaces, minimizing undesirable flow or migration, thereby avoiding the formation of bare or uncoated surface areas. The particular materials and conditions for applying the carbon film are more fully described in U.S. Pat. No. 2,092,557, the substance of which is incorporated herein by reference.
In any event, conventional cleaning treatments of the types heretofore known in the art have been unsuccessful in appreciably removing the minute carbon particles embedded in the copper layer as a result of the solidification of the molten copper during the cooling stage of the brazing operation. The present invention overcomes many of the problems and disadvantages associated with prior art cleaning techniques by providing a method which is quick, efficient and effective to remove substantially all of the residual carbon contaminants from the surfaces of copper brazed multi-ply steel tubing, enabling increased production capacity and improved results in the corrosion resistance of the electroplated and terne coated tubing due to the improved uniformity and continuity of the copper and/or nickel electroplatings deposited thereon.